This relates generally to displays, and, more particularly, to displays such as liquid crystal displays.
Displays are widely used in electronic devices to display images. Displays such as liquid crystal displays display images by controlling liquid crystal material associated with an array of image pixels. A typical liquid crystal display has a color filter layer and a thin film transistor layer formed between polarizer layers. The color filter layer has an array of pixels each of which includes color filter subpixels of different colors. The thin film transistor layer contains an array of thin film transistor circuits. The thin film transistor circuits can be adjusted individually for each subpixel to control the amount of light that is produced by that subpixel. A light source such as a backlight may be used to produce light that travels through each of the layers of the display.
A layer of liquid crystal material is interposed between the color filter layer and the thin film transistor layer. During operation, the circuitry of the thin film transistor layer applies signals to an array of electrodes in the thin film transistor layer. These signals produce electric fields in the liquid crystal layer. The electric fields control the orientation of liquid crystal material in the liquid crystal layer and change how the liquid crystal material affects polarized light.
An upper polarizer is formed on top of the display and a lower polarizer is formed on the bottom of the display. As light travels through the display, the adjustments that are made to the electric fields in the liquid crystal layer are used to control the image that is displayed on the display.
The liquid crystal layer in liquid crystal displays exhibits a natural birefringence. In a birefringent layer, the index of refraction of the layer varies as a function of the polarization of incoming light. If left uncompensated, the birefringence of the liquid crystal layer in a display can adversely affect off-axis image quality and can therefore limit viewing angle.
Off-axis display performance can be improved by including a birefringent compensating layer in a liquid crystal display. The compensating layer, which is typically located between the lower polarizer and the thin film transistor layer, has birefringent properties that counteract the effects of the liquid crystal layer. Birefringent compensating layers are often formed from polymerized liquid crystals and may exhibit indices of refraction of about 1.5 to 1.8.
Signals may be applied to the electrodes in the thin-film-transistor layer using a grid of signal lines. The signal lines may include orthogonal sets of gate lines and data lines. The gate lines may be connected to the gates of thin film transistors that drive signals onto the electrodes. The data lines may distribute power supply voltages.
The electrodes in the thin film transistor layer are generally formed from a transparent conductive material such as indium tin oxide. The electrodes are conductive, so that they can be used to apply an electric field to an associated image pixel region of the liquid crystal layer. The electrodes are transparent so that light that passes through the image pixel regions will not be blocked by the presence of the electrodes.
To ensure that the signal lines in the thin film transistor layer are sufficiently conductive to effectively distribute signals to the thin film transistors and their associated electrodes, the signal lines are generally formed from metal. Metals have a significantly higher conductivity than transparent conductive materials such an indium tin oxide and are satisfactory when used to distribute control signals to the thin film transistors and electrodes on the thin film transistor layer.
However, the use of metal in forming the grid of signal lines in a liquid crystal display can lead to unsightly visible artifacts due to optical interference. These artifacts, which are sometimes referred to as curtain mura, result from the Moiré effect. Due to the presence of high index materials beneath the grid lines such as birefringent compensating films, there is an index of refraction discontinuity beneath the grid lines. This discontinuity forms a reflective interface that can reflect and image of the underside of the grid lines towards a viewer. The grid lines and the pattern of the reflected image of the underside of the grid lines can optically interfere with each other, leading to an undesirable Moiré pattern that is visible to a user of the display. Even though the pitch of the grid lines may be small (e.g., tens of microns), the bands of dark and light lines in the Moiré pattern can be one or more centimeters in width and can therefore interfere significantly with a user's ability to discern images on the display.
It would therefore be desirable to be able to provide improved displays such as displays that exhibit minimized curtain mura.